1. Technical Field
The present invention relates to a trunk circuit for use in a private automatic branch exchange (PABX), and more particularly to a Direct Inward-Outward Dialing (DIOD) trunk circuit for bidirectional interfacing between a private automatic branch exchange (PABX) and a central office exchange (CX).
2. Related Art
Private automatic branch exchange (PBX) systems as described, for example, in U.S. Pat. No. 4,028,498 for Private Automatic Branch Exchange System And Apparatus issued to Mehaffey et al, U.S. Pat. No. 4,941,171 for Electronic Private Branch Exchange issued to Hasegawa, and U.S. Pat. No. 5,291,549 for Private Branch Exchange And Line Exchange Method issued to Izumi, are well known to utilize the step-by-step and crossbar switching techniques for providing switched communications between a plurality of extension subscribers and a central office exchange (CX). Trunk circuits are required to interface a number of trunk lines from the central office exchange (CX) to a number of telephone lines within the private automatic branch exchange (PBX). Typical trunk circuit can be application specific such as direct inward dialing (DID) trunk, direct outward dialing (DOD) trunk, signaling trunk and operator access trunk, or universal in nature to provide all of the necessary features as disclosed, for example, in U.S. Pat. No. 3,748,396 for Direct Inward Dialing Trunk Circuit issued to Hestad et aL, U.S. Pat. No. 3,806,660 for Trunk Circuit Having Selective Interface Combinations issued to Gueldenpfennig et al., U.S. Pat. No. 4,049,919 for Communication System Trunk Circuit issued to Young, U.S. Pat. No. 4,310,725 for Interface For Private Branch Exchange issued to Mehaffey, U.S. Pat. No. 4,776,007 for Solid State Trunk Circuit issued to Styrna et al., U.S. Pat. No. 4,894,860 for Trunk Interface Circuitry For Telephone Systems issued to Korsky et al., and U.S. Pat. No. 4,914,690 for Universal PBX Interface issued to Hagedorn.
For example, a DID trunk such as disclosed in U.S. Pat. No. 3,748,396 is used to allow a calling subscriber from a distant office to call a predetermined extension of the PABX through the central office exchange (CX) directly without an operator. When a calling subscriber dials the extension of the called subscriber of the private automatic branch exchange (PABX), the central office exchange (CX) downloads the extension of the called subscriber on a DID trunk and the PABX receives the extension number to route the call directly to the called subscriber. However, the DID trunk cannot be seized for direct outgoing calls from extension subscribers of the PABX to a distant office through the central office exchange (CX). A loop start trunk can be used for outgoing calls from extension subscribers of the PABX to a distant office through the central office exchange (CX). Both the DID trunk and loop start trunk can be implemented for handling incoming calls and outgoing calls. However, the combined trunk circuit requires construction of large inductance coils exhibiting high impedance and capacitors exhibiting high capacitance which are bulky and cost. A special trunk circuit, known as a direct inward-outward dialing (DIOD) trunk is circuit, can also be constructed for automatic operation on both incoming and outgoing calls such as disclosed in the IBM Technical Disclosure Bulletin, Vol. 11, No. 1, Jun. 1968.
Conventional DIOD trunk circuit which is commonly incorporated into a private automatic branch exchange (PABX), is illustrated in FIG. 1. This private automatic branch exchange (PABX) is exemplary of an European PTT ALS-70 system which generally includes a central processing unit (CPU) 10, a memory 20, an interface circuit 30, at least an extension line subscriber circuit 40, a switching circuit 50, a tone signal generator 60, a dual-tone multi-frequency (DTMF) receiver 70, a DTMF transmitter 80, a DIOD trunk circuit 90, a tie line circuit 100, an extension subscriber telephone 120 and a ring generator 130 connected to the extension line subscriber circuit 40. The CPU 10 controls call switching operations of incoming and outgoing calls via the DIOD trunk circuit 90 connected to a central office line and the tie line circuit 100, and provides a variety of services to a system user. Memory 20 stores a program that enables the CPU 10 to perform call processing and a variety of other system functions. Switching circuit 50 switches various tone signals and voice signals under the control of the CPU 10. Subscriber circuit 40 transmits and receives a variety of tone signals and voice signals to and from an extension telephone 120 of an extension subscriber, selectively connects the extension telephone 120 to a central office C.O. line through the DIOD trunk circuit 90, supplies power to the extension telephone 120, and also acts as an interface between the extension telephone 120 and the switching circuit 50. Ring generator 130 generates a ring signal that is supplied to the extension telephone 120 through the subscriber circuit 40. Tone signal generator 60 generates various tone signals that are supplied to the switching circuit 50 under the control of the CPU 10. DTMF receiver 70 is connected to the switching circuit 50 for analyzing a dual-tone multi-frequency (DTMF) signal generated from the extension telephone 120. DTMF transmitter 80 generates and supplies the DTMF signal to the switching circuit 50 under the control of the CPU 10. The interface circuit 30 serves to interface the subscriber circuit 40, the tone signal generator 60, the DTMF receiver 70, the DTMF transmitter 80, the DIOD trunk circuit 90 and the tie line circuit 100 to the switching circuit 50.
FIG. 2 illustrates the conventional DIOD trunk circuit 90 which is connected to a central office C.O. line from a central office exchange (CX) and serves as an interface to the private automatic branch exchange (PABX). As shown in FIG. 2, the conventional DIOD trunk circuit 90 is constructed of an overvoltage protection circuit 91, a high impedance generation circuit 92, a backward loop sensing circuit 93, a forward loop sensing circuit 94, a large core transformer 95, and a series of large size coils L1 and L2 with high impedance and large size capacitors which are comparably expensive and add to the cost of the unit. For example, in order to adjust the balance of a transmission signal on the telephone line, capacitors C3 and C4 must have capacitance values of at least 1 xcexcF/100V. The requirement of large inductance coils and high capacitors, as I have observed, is cost prohibitive and bulky. Accordingly, an improved DIOD trunk circuit for interface between a private automatic branch exchange (PABX) and a central office exchange (CX) can still be contemplated.
Accordingly, it is an object of the present invention to provide an improved trunk circuit for interface between a private automatic branch exchange (PABX) and a central office exchange (CX).
It is also an object to provide a direct inward-outward dialing (DIOD) trunk circuit for a private automatic branch exchange (PABX) designed to minimize the use of coils and capacitors with high capacitance in order to reduce cost and increase production.
It is another object of the present invention to provide a direct inward-outward dialing (DIOD) trunk circuit for a private automatic branch exchange capable of handling incoming direct inward dialed (DID) telephone calls and outgoing direct outward dialed (DOD) telephone calls without an operator.
These and other objects of the present invention can be achieved by a direct inward-output dialing trunk circuit which comprises a tip T and ring R terminals which are connected to a central office line with a battery source of xe2x88x9248 volts D.C. potential; and first and second transformers T1 and T2 connected in series and having a primary coil respectively connected to the tip T and ring R terminals and a secondary coil connected to a switching circuit 50 of the private automatic branch exchange (PABX) via a speaking circuit 102 for impedance matching and for forming a speech path between a central office line subscriber and a corresponding PABX extension subscriber 120 of the PABX. A tone signal detecting circuit 104 is connected to the speaking circuit 102 for detecting a dial tone generated by a tone supply circuit 60 in response to both incoming and outgoing calls. A DIOD signal detecting/sending circuit 101 is connected to the tip T and ring R terminals for detecting the polarity of the xe2x88x9248 volts D.C. potential at the tip T and ring R terminals and the current flowing through the tip T and ring R terminals to determine reception of the incoming and outgoing calls and to enable formation of the speech path between the central office line subscriber and the corresponding PABX extension subscriber 120 of the PABX via the central office line to the interface circuit 30. A switch SW1 is connected to the transformers for selectively enabling the current to flow between the tip T and ring R terminals at the central office line through the DIOD signal detecting/sending circuit 101 via either a low impedance loop or a high impedance loop comprised of a serially connected resistor R1 and diode D1, wherein the current passing through the low impedance loop from tip T terminal to ring R terminal at the central office line alerts the central office network or the PABX to form the speech path between the central office line subscriber and the corresponding PABX extension subscriber in response to one of incoming and outgoing calls, and wherein the current passing through the high impedance from ring R terminal to tip T terminal at the central office line alerts the CPU 10 to control a ring generator 130 to generate ringing tones to the calling subscriber from either the central office line or the PABX extension line. A relay drive circuit 107 is installed to control the switch SW1 by way of a relay coil L1 to select either the low impedance loop or the high impedance loop for the current to flow through the DIOD signal detecting/sending circuit 101 between tip T terminal and ring R terminal at the central office line.
The DIOD signal detecting/sending circuit includes a DIOD signal detecting unit and a DIOD signal sending unit. The DIOD signal detecting unit is comprised of an operational amplifier OP1 having an inverting terminal 6 connected to the tip T terminal at the central office line and a non-inverting terminal 5 connected to the ring R terminal at the central office line; a pair of transistors Q11 and Q12; and a photo-coupler PT3, for sensing the voltage polarity of the tip T and ring R terminals upon seizure of the PABX to establish the speech path between the central office line subscriber and the corresponding PABX extension subscriber when tip T terminal exhibits positive D.C. potential and ring R terminal exhibits negative D.C. potential and for providing a high impedance polarity inverting signal supplied from the central office line at a high impedance output terminal 14 to the CPU 10 via the interface circuit 30 to control the tone supply circuit 60 to generate the dial tone in response to, for example, an incoming call. When the voltage potential between tip T and ring R terminals is reversed, however, the DIOD detection circuit remains idle.
The DIOD signal sending unit is comprised of a series of diodes D11, D12, D13, D24, a pair of photo-couplers PT1, PT2 with an input terminal 4 for receiving and allowing the current flowing from tip T terminal via either the low impedance loop or the high impedance loop selected by switch SW1 to be current limited by a current limit circuit 121 and subsequently output to the ring T terminal at the central office line via a backward supply terminal 1. For example, upon seizure of the PABX in response to an incoming call, the battery provides positive D.C. potential to tip T terminal and negative D.C. potential to ring R terminal at the central office line so that current can flow from tip T terminal via the low impedance loop selected by switch SW1 and is limited by the current limit circuit 121 before reaching to ring R terminal in a low impedance state alerting the central office network to form a speech path between the central office line subscriber and the corresponding PABX extension subscriber. After the telephone number is dialed, switch SW1 switches to the high impedance loop so that the current flowing through the DIOD signal detecting/sending circuit 101 and the current limit circuit 121 before reaching to ring R terminal is in a high impedance state alerting the CPU 10 to control the ring generator 130 to generate ringing tones to the calling subscriber.
Lastly, an interface circuit 30 used to interface the DIOD trunk circuit 90 with the switching circuit 50 of the private automatic branch exchange (PABX) may be comprised of first, second and third latch circuits 105, 106, 107 for latching the dial tone detected by the tone signal detecting circuit 104, the polarity inverting signal applied from the central office line and the busy signal generated from the DIOD signal detecting/sending circuit 101 to the CPU 10; a control circuit 109 for driving the relay drive circuit 107 to control switch SW1 to select current flowing through either a high impedance loop or a low impedance loop under control of the CPU 10; and a buffer circuit 110 installed to buffer communication between the DIOD trunk circuit 90 and the CPU 10 of the private automatic branch exchange (PABX).
The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example.